Both norepinephrine (NE) and 5-hydroxytryptamine (5-HT) contribute to and may have a pathogenetic role in vascular disease. Therefore the receptors on which these substances act are critical as sites of therapeutic intervention. Data are presented in this proposal demonstrating that 5-HT acts on serotonergic receptors in some blood vessels, but predominantly on Alpha-adrenoceptors in certain other blood vessels. These 5-HT-sensitive Alpha-receptors resemble the recently identified vascular postsynaptic Alpha2 receptor, and exhibit a marked variation in density and distribution among vascular beds. On the basis of these observations, it is hypothesized that there are at least two major subtypes of Alpha-receptors in blood vessels and that there are marked differences in the density and distribution of these receptors. In order to test this hypothesis, rabbit and dog vessels as well as human vessels from surgery and autopsy will be obtained from all major vascular beds. They will be cleaned, cut into 3mm rings and mounted in tissue baths for the measurement of isometric tension development. Dose-response curves to Alpha-and serotonergic agonists will be obtained with and without pretreatment with phenoxybenzamine to cause partial receptor inactivation. Doseresponse curves will also be obtained in the presence and absence of specific Alpha- and serotonergic receptor antagonists. The following parameters will be calculated from these data: potency (ED50), agonist (KA) and antagonist (pA2 or KB) dissociation constant and the relationship between fraction of receptors occupied by agonist (RA/Rt) and magnitude of contractile response. The adrenergic agents used will allow differentiation between postsynaptic Alpha1 and Alpha2-receptors. The specific aims of the study are to: 1) determine orders of agonist potency and affinity (1/KA) and antagonist pA2 values at vascular Alpha- and serotonergic receptors; 2) establish the relative contribution of Alpha- and serotonergic receptors to the contractile response to 5-HT in each blood vessel studied and 3) determine the relative densities of vascular Alpha- and serotonergic receptors. Interpretation of density will be based on the extent of spare receptors for full agonists, the ratio of maximal responses of partial to full agonists and RA/Rt at threshold contractile response. The main goal of the proposed project is to carry out pharmacological "receptor mapping" to identify the localization and density of alpha1-, Alpha2-, 5-HT-sensitive-Alpha- and serotonergic receptors in the blood vessels of rabbit, dog and human. This knowledge will provide the basis on which to formulate more specific therapeutic strategies in the treatment of hypertension and vasospastic disorders.